A color scan of the original computer printout of the "Wow!" signal as detected by the Big Ear Radio Observatory in 1977.

Credit: Big Ear Radio Observatory and North American Astrophysical Observatory (NAAPO)

An astronomer thinks he's pinpointed the source of a mysterious radio signal from space: a passing comet that nobody knew about. But his colleagues said they're still skeptical of the explanation, noting that comets don't emit radio waves in the right way.

Antonio Paris, an astronomer at St. Petersburg College in Florida, recently published a paper in the Journal of the Washington Academy of Sciences saying the mysterious "Wow! signal," a truly bizarre radio signal detected almost 40 years ago, seems to match up with the location of a comet called 266P/Christensen that hadn't been cataloged at the time. (The comet was discovered more recently, in 2006. Originally, Paris' hypothesis was that a second comet might also be the culprit, one called P/2008 Y Gibbs.) Explanations for the Wow! signal have ranged from intermittent natural phenomena, to secret spy satellites, to, yes, aliens.

Others aren't so sure. "We do not believe the two-comets theory can explain the Wow! signal," Jerry Ehman, the astronomer who discovered the Wow! signal in 1977, told Live Science. [5 Times We Thought We Found Aliens]

Wow! signal

The Wow! signal's name comes from just how striking and strange it was. The radio signal appeared on the night of Aug. 15, 1977, when it was picked up by the Big Ear radio telescope at The Ohio State University. It lasted 72 seconds. It was "loud" — more intense than anything in the background sky that night. It was also a narrow-bandwidth signal; the range of frequencies it covered was small, similar to those of artificial signals. AM radio, for example, has channels that are only 10,000 cycles above or below the designated frequency on the dial. Further, the signal was at a frequency of about 1,420 megahertz (MHz), also called the 21-centimter line. That's the same frequency as radio waves emitted by neutral hydrogen gas in space. It's a region that is relatively free of noise from other objects, and one researchers involved in the search for extraterrestrial intelligence have been interested in for a long time because it could be used for interstellar transmissions.

The signal did not repeat, and subsequent attempts to find it proved fruitless. Ehman marked "Wow!" in red pen on a printout that shows the numbers representing the signal.

Back in 1977, the now-dismantled Big Ear telescope was looking for alien signals, in an early iteration of the search for extraterrestrial intelligence, or SETI. But no one expected to see anything like the Wow! signal, and the Big Ear telescope heard nothing like it again.

Without a repeat signal, it was impossible to tell what it was; even getting a precise location wasn't easy because the signal was short-lived. Ehman, now retired, told Live Science that, beyond a certain distance, it's hard to tell how far away a radio signal is coming from.

Comet signature

In his paper, Paris wrote that comets will, under certain conditions, emit radio waves from the gases that surround them as they zoom closer to the sun. According to the study, Comet 266P/Christensen was in about the right position on the right day in 1977. Paris first floated the idea in early 2016, and proposed a program of using radio telescopes to listen for the emission of such radio waves. [Face on a Comet: Ghostly Faces in Space]

The comet project had three phases. "The first phase was the hypothesis, which led to the second phase: Do comets emit 1,420 [MHz signals]? It appears yes, they do," Paris told Live Science.

In the third phase, set for 2018, Paris plans to explore the mechanisms of the emissions —why comets should generate radio waves at that particular wavelength. Paris said little research has been done on the topic.

"There have been a handful of studies, but I suspect we are the first to specifically build a 10-meter radio telescope to specifically look at this type of solar system body," he said.

To see if a signal could have come from comets, Paris first used a radio telescope to look at the sky in the region of the Wow! signal. With this step, he wanted to see what the background looked like at the relevant frequency. He also checked two other comets to be sure that they did, in fact, emit radio signals at the 1,420-MHz frequency, and found that they did.

Then, in January, Paris directed the radio telescope to point at Comet 266P/Christensen as it passed through the region of the sky where the Wow! signal was seen. (Comet 266P/Christensen has an orbital period of about 6.65 years, and its apparent location in the sky will vary depending on where Earth is in its own orbit around the sun. The comet passed near, but not exactly, where the Wow! signal was — about 2 degrees north of the Wow! signal location.

Skepticism abounds

Yet several astronomers, including Ehman, think Paris is wrong about the comet. Ehman looked at Paris' study with Robert Dixon, who directs the radio observatory at The Ohio State University (Big Ear was destroyed in 1997). Two big issues are that the signal didn't repeat, and it appeared for such a short time. Ehman noted that the Big Ear telescope had two "feed horns," each of which provides a slightly different field of view for a radio telescope. [5 Huge Misconceptions about Aliens]

"We should have seen the source come through twice in about 3 minutes: one response lasting 72 seconds and a second response for 72 seconds following within about a minute and a half," Ehman told Live Science. "We didn't see the second one."

The only way that can happen, he said, is if the signal was cut off abruptly. A comet wouldn't produce that kind of signal, because the gases that surround them cover large, diffuse areas. Nor would the comet have escaped from the radio telescope's field of view that fast.

But Ehman isn't convinced it's aliens, either. There are many phenomena that show sudden appearances and disappearances of radio signals, including fast radio bursts (FRBs), which are mysterious radio bursts with hotly-debated astrophysical origins that generate irregular signals that last only milliseconds. If the the Big Ear picked up only the tail end of such an emission, the data could look similar to the Wow! signal, Ehman speculated.

"The issue with the feed horns is something no one can explain, including me," Paris said. "There is some data out there to suggest the issue is at the telescope end and not the phenomenon itself." So it's possible that the signal could have been caused by a glitch in the Big Ear telescope.

The other issue is the frequency of transmission. Paris said he has shown that comets can emit in that range, but Seth Shostak, a senior astronomer at the SETI Institute, is skeptical. Shostak used to study emissions from neutral hydrogen in the 1,420-MHz range, and is less sure the emission would look right. Comets may not generate enough hydrogen to make a bright enough signal like Wow!.

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Jesse Emspak is a contributing writer for Live Science, Space.com and Toms Guide. He focuses on physics, human health and general science. Jesse has a Master of Arts from the University of California, Berkeley School of Journalism, and a Bachelor of Arts from the University of Rochester. Jesse spent years covering finance and cut his teeth at local newspapers, working local politics and police beats. Jesse likes to stay active and holds a third degree black belt in Karate, which just means he now knows how much he has to learn.